1. Field of the Invention
This invention relates to an hetero-junction bipolar transistor and a manufacturing method of the transistor.
2. Related Prior Art
In a process for manufacturing a semiconductor device, in which silicon (Si) is a primary material, a passivation film made of inorganic material such as a silicon dioxide (SiO2) may be applied for protecting and covering the device from an ambience. The SiO2 with excellent quality can be easily obtained by oxidization of silicon itself. In the case of a hetero-junction bipolar transistor (hereinafter denoted as HBT) primarily made of compound semiconductor materials, an oxide material with a good quality as compared with the thermally oxidized silicon film has not been realized yet.
The HBT made of compound semiconductor material generally has a base-collector junction, an edge of which is exposed to the ambience. When an inorganic material, such as SiO2 or a silicon nitride (SiN) covers the edge of the junction, a current leaking from the collector to the base may increase because a crystal quality is deteriorated by the existence of the inorganic film at the edge of the junction. When the semiconductor surface is exposed to the plasma to form the inorganic film thereon, plasma damages will be introduced. Moreover, when the inorganic film covers the semiconductor surface, surface states due to dangling bonds of the semiconductor will increase. These damages and surface states may provide a current leaking pass.
Under a normal operating conditions of the HBT, large reverse bias is applied between the base and the collector. Therefore, to decrease the leak current between the base and the collector is most effective to enhance the breakdown voltage. In the HBT, in which the inorganic film covers the surface thereof, it has been difficult to decrease the leak current between the base and the collector, and to enhance the breakdown voltage.